ENCLOSING DAMS. 57 



a chord of 467 feet and a versed line of 76 feet. The width at 

 the top was 18 feet, and at the ground line 23 feet. The 

 extreme height from the toe of the outer row of piles to the 

 top of the dam was 61 feet 6 inches. The dam was formed 

 with two rows of whole piles battering both ways, the outer 

 row in lengths of 35 to 40 feet, 12 inches wide by 14 inches 

 thick, and driven close together, as sheet-piling, with the toes 

 reaching to 35 feet below the level of the old dock sill. The 

 piles in the inner row were from 30 to 35 feet long, of the same 

 width and thickness, driven also as sheet -piles, with the toes 

 reaching to 30 feet below the same datum. 



On every fourth pile of both the outer and inner rows was 

 placed an upright, 12 inches wide by 14 inches thick, by means 

 of a scarf 4 feet long, secured by four 1-inch bolts, and reaching 

 to the level of 3 feet above high- water equinoctial spring tides, 

 the whole being secured by five tiers of wales, placed on the 

 outside of the piles and uprights, at vertical distances of about 

 8 feet. Through these wales were passed, at longitudinal 

 distances of 12 feet, a series of 2-inch wrought-iron tie-rods, 

 screwed up at each end with hexagonal nuts on cast-iron washer 

 plates 9 inches square by 2 inches thick. 



At the level of high- water spring tides an inner wale 12 

 inches square was fixed on both the inner and outer rows of 

 uprights, to which diagonal horizontal struts or braces, 12 inches 

 square, were bolted for the purpose of keeping the uprights 

 asunder to the proper width at the top. 



The whole interior of the dam on both the inner and outer 

 rows of uprights from the level of the old dock sill to the top 

 was sheeted with horizontal planking 3 inches in thickness to 

 hold the puddle. 



The excavation within the dam was taken down to 18 feet 

 below the old dock sill before the puddle was put in. The 

 ground upon which the dam rested was chiefly fine sand ; this, 

 when saturated with water, became a quicksand. 



The timber specified to be used was crown Memel, but it was 

 ascertained as the work proceeded that over a considerable portion 

 of the site Memel could not be used owing to the compactness of 

 the sand ; American elm piles were therefore substituted. 



Excavation within the dam was a difficult operation, re- 

 quiring to be done by tide work in short lengths. 



After the dam had been filled with puddle to the height 



